The long-term goal of this proposal is to understand the genetic basis of focal segmental glomerulosclerosis (FSGS) in humans. The underlying hypothesis of this work is that understanding the genetic basis of FSGS will yield considerable insight into the biologic pathways responsible not just for inherited forms of FSGS, but also secondary FSGS, as well as glomerulosclerosis and proteinuria resulting from common initiators such as diabetes and hypertension. The major aim of the first period of this award was to identify the FSGS gene on chromosome 19. We have identified this gene as ACTN4, encoding the cytoskeletal protein ctactinin- 4, and have demonstrated that dominant mutations lead to increased binding to actin filaments, progressive proteinuria, and renal insufficiency. We have identified, clinically characterized, and collected DNA samples from a large number of families and individuals with FSGS. The study of samples from these subjects forms the basis of this work. There are three basic aims of this proposal: First, we will perform mutational analysis of ACTN4, NPHS2, NPHS 1 in families with FSGS and patients with sporadic FSGS. Towards this aim, we will continue our ascertainment of families with FSGS, as well as sporadic adult and pediatric cases. Mutational analysis will include evaluation for nucleotide changes causing disease under Mendelian and oligogenic models of inheritance. Second, we will assess the role of common variation in Mendelian FSGS/NS genes ACTN4, NPHS2, NPHS 1 in FSGS. We will define the haplotype structures of FSGS/NS genes ACTN4, NPHS 1, and NPHS2 by definition of common single-nucleotide sequence polymorphisms (SNPs). We will analyze the contribution of these variants to the FSGS phenotype using association studies and inheritance-based approaches. Thirdly, we aim to identify new FSGS genes. We will use a candidate gene approach, informed by recent advances in podocyte genetics and biology, to identify additional genes mutated in familial forms of FSGS. As an alternative approach, we will use a genome-wide linkage analysis to identify new loci.